Processes for removing sulphur from coal, or for extracting desirable metals from an ore body, are well-known. Coal desulfurization is highly desirable in that it is believed that combustion of coal, and specifically high-sulphur content coals, is a contributing factor to acid rain and other environmental problems. Numerous methods for desulfurizing coal have been attempted. Physical separation, bacterial oxidation and chemical processes can all be demonstrated to produce the desired effect in laboratory or pilot plant scale tests. However, each of these processes suffers because they are either too expensive or too difficult to accomplish on the massive scale involved. Physical separation processes are difficult because of the small particle size necessary. Bacterial oxidation suffers because of the long exposure times required (on the order of 5-15 days). Chemical processes suffer from the expense of the large quantity of chemicals required.
For example, U.S. Pat. No. 4,775,627 discloses a process whereby pyrite particles and high-sulphur content coal are modified so as to be more hydrophyllic, and more easily separated by conventional means, such as froth flotation. Ground coal particles are preconditioned by subjecting the pyrite to thiophilic bacteria adapted to the process, permitting the bacteria to alter the hydrophobicity of the pyrite particles. U.S. Pat. No. 4,822,413 discloses a process to extract metals from an ore containing one or more metallic sulphides using a leach liquor containing bacterially-generated ferric ions in order to effect metal sulfide dissolution. The metals removed are capable of dissolution in an acidic ferric sulfate solution (such as copper) or are more easily removed by subsequent processing (such as gold) as a result of exposure to the acidic ferric sulfate solution. Suitable bacteria, such as Thiobacillus ferrooxidans may be used to increase dissolution of metals in certain circumstances.
It has been proposed in U.S. Pat. No. 4,043,884 to upgrade the kerogen components of oil shale by leaching carbonate materials from the oil shale to produce a porous residue, forming a slurry of the residue with a reductive electrolytically active solution, and then subjecting the slurry to reductive electrolysis. The reduced residue is then more easily separated from the electrolyzed slurry.
The phenomenon of bacterio-electric leaching of metals from minerals combines two processes having a common feature--the bacterial oxidation of Fe.sup.+2 in an acid solution. The first process is the oxidative leaching of pyritic minerals by iron-oxidizing bacteria (most commonly thiobacilli)--the so-called "pyrite cycle". The second process is the acceleration of the oxidation of reduced iron by iron-oxidizing thiobacilli by subjecting the process to an applied electric potential and the resulting current.
The pyrite cycle is a chain reaction in which ferric ions produced by bacterial oxidation react with the sulphur of a pyritic mineral in order to oxidize the pyrite. This oxidation releases ferrous ions, enabling the chemo-lithotrophic oxidation of the pyrite, and sustains the pyrite decomposition by regenerating Fe.sup.+3 as the oxidizing agent. Pyrite oxidation and solubilization are limited by the concentration of ferric ions, and their accessibility to the mineral substrate. Both the concentration and accessibility of Fe.sup.+3 ions are affected by the precipitation of bacterially-oxidized iron from solution as an Fe.sup.+3 sulfato complex.
Precipitation usually occurs initially in the form of a metastable amorphous hydrated ferric sulfate. As illustrated in FIG. 1, the presence of jarosite directing cations and excess sulfate causes the Fe.sup.+3 sulfato complex to be converted to a stable crystalline jarosite. Under more alkaline conditions the deposits comprise iron oxides and oxyhydroxides that are distinguishable from bacterial oxidation products.
Iron oxidizing bacteria are essential to the pyrite cycle by regenerating the soluble Fe.sup.+3 lixiviant under acid conditions permitting extraction of metals from sulfide minerals and coal--conditions under which abiotic auto-oxidation of iron occurs slowly.
The acceleration of electro-oxidation of reduced iron by iron oxidizing thiobacilli occurs when an electro-oxidizable metal mineral liberates Fe.sup.+2 under the influence of an electric potential. As illustrated in FIG. 2, in the presence of iron-oxidizing thiobacilli and in an acid environment, the flow of electric current and solubilization of iron is accelerated through depolarization of cathodic sites by bacterially oxidized iron in solution. When Fe.sup.0 corrodes in an acidic aqueous environment, Fe.sup.+2 passes into solution as electrons are transferred from Fe.sup.0 at anodic sites to cathodic sites on the metal. The cathodic sites must be depolarized by a loss of electrons to an oxidizing agent in order for the process to continue. In an acid environment, hydrogen ions may serve as the oxidizing agent, however when Fe.sup.+3 is present in such environment, it competes at the cathodes with H.sup.+ for available electrons. If the Fe.sup.+2 produced is oxidized by the thiobacillus, a chain reaction of iron oxidation and reduction occurs analogous to that of the pyrite cycle.